Security paper adapted to resist fraudulent alteration of toner-printed information thereon and to make such alteration or attempts at alteration evident has a maximum Scott-Bond strength of 150 J m.sup.-2; a Bendtsen roughness of not more than about 150 ml min.sup.-1; and is size press or otherwise treated with a thermally-softenable polymer composition, for example acrylic polymer or copolymer, a polyvinyl acetate polymer, a vinyl acetate/ethylene copolymer, or a vinyl acetate/vinyl chloride/ethylene terpolymer. Preferably the paper has filler content of about 15% to about 20% by weight.